The present invention relates to a method and an apparatus for detecting a photoacoustic signal to detect information relative to the surface and the subsurface of a sample using photoacoustic or photothermal effect, and more particularly to a method and an apparatus for detecting a photoacoustic signal, devised to effectively correct abnormality of phase shifts by phase jumps at specified points of the sample when a photoacoustic signal is detected at those points of the sample.
The photoacoustic effect was discovered by Tyndall, Bell, Roentgen, et al. in 1881. As shown in FIG. 2, when intensity-modulated light (intermittent light beam) 19 is irradiated to a sample 7 by focusing the light as an excitation light with a lens 5, heat is generated in a light absorption region V.sub.OP 21, and periodically diffused through a thermal diffusion region V.sub.th 23 defined by a thermal diffusion length .mu..sub.S 22 so that the resulting thermal distortion wave gives rise to a thermoelastic wave (ultrasonic wave). By detecting this ultrasonic wave, i.e. a photoacoustic wave by a microphone (acousto-electric converter) or by a piezo-electric or a light interferometer to thereby obtain a signal component synchronized with the modulated frequency of the excitation light, information relative to the surface and subsurface of the sample can be obtained. Incidentally, the thermal diffusion length .mu..sub.S 22 can be obtained by the following expression (1) from the thermal conductivity k, density .rho., and specific heat c of the sample 7 when the modulated frequency of the excitation light is denoted by f.sub.L. ##EQU1##
A technique for detecting the above photoacoustic signal is disclosed, for example, in "HIHAKAI KENSA", Vol. 36, No. 10 issue, pp. 730-736 October 1987 (Showa 62) or IEEE 1986 ULTRASONIC SYMPOSIUM pp. 515-526 (1986).
Referring to FIG. 1, one example of such a technique will be explained. A parallel light beam emitted from a laser 1 is intensity-modulated by an acousto-optical modulator (A0 modulator) 2. The thus obtained intermittent light is expanded to a parallel beam of a desired diameter by a beam expander 3, which is reflected by a half mirror 4, and then focused by a lens 5 on the surface of the sample 7 placed on an X-Y stage 6. The thermal distortion wave emanating from the focusing position 21 on the sample 7 generates a thermoelastic wave, thus causing minute displacements at the surface of the sample. The minute displacements will be detected by a Michelson interferometer explained below. After the parallel light beam from the laser 8 is expanded to a desired beam diameter by the beam expander 9, the beam is separated into two beams traveling along two optical paths by a beam splitter or a half mirror 10. One is focused at the focusing position 21 on the sample by the lens 5, while the other is irradiated to a reference mirror 11. Then, the light reflected from the sample 7 and the light reflected from the reference mirror 11 interfere with each other at the half mirror 10. The interference light is focused by a lens 12 on a photoelectric converting element 13 such as a photodiode by a lens 12 to provide a photoelectric-converted interference intensity signal. After amplified by a preamplifier 14, the interference intensity signal is sent to a lock-in amplifier 16. Using a modulation frequency signal from an oscillator 15 used to drive the acousto-optical modulator 2 as a reference signal, the lock-in amplifier 16 extracts only the modulated frequency component contained in the interference intensity signal. This frequency component has information relative to the surface or the inside of the sample 7. According to the expression (1), by varying the modulated frequency, the thermal diffusion length .mu..sub.S 21 can be changed and information as to the condition through the depth of the sample can be obtained. If there is a defect such as a crack in the thermal diffusion region V.sub.th 23, the amplitude of the modulated frequency component in the interference light intensity signal and the phase thereof relative to the modulation frequency signal change, by which the presence of the defect can be known. An X-Y stage shifting signal and an output signal from the lock-in amplifier 16 are processed by a computer 17. Accordingly, the photoacoustic signal corresponding to the respective points on the sample can be gathered and displayed as a two-dimensional image on a display 18 such as a monitor television.
Though the above-mentioned prior-art technique is extremely effective means for detecting a photoacoustic signal in non-contact and non-destructive inspection of samples, but has the following problems.
In the conventional photoacoustic detection optical system as shown in FIG. 1, when a two-dimensional internal information of a sample is to be obtained, it is necessary to perform a two-dimensional scanning of the surface of a sample by a relative motion of two beams, that is, an excitation light for generating a photoacoustic effect and a probe light for detecting minute displacements of the sample surface caused by the photoacoustic effect. This two-dimensional scanning is the so-called point scanning by which information is obtained point by point, and therefore, if one tries to scan the whole surface of the sample, a very large amount of detection time is required. This necessity for a large amount of detection time is the greatest reason why the photoacoustic detection technique has not been applied to internal defect inspection of samples in the production line. In some samples, the reflectance of the surface varies with different positions of the sample. In such a case, with the prior-art technique, the intensity of the reflected light of the probe light unavoidably contains information relative to the surface reflectance in addition to information about the internal condition, so that it is difficult to accurately detect only information about the inside of the sample. Furthermore, in some samples, the surface is not flat and has local undulations. In this case, in the prior-art technique, the phase of the reflected beam of the probe light varies according to the undulations of the sample surface, so that the reflected light intensity includes information with regard to the surface undulations in addition to internal information and, as a result, it is difficult to accurately detect only internal information about the internal condition.
Moreover, in the conventional photoacoustic signal detection method, there is no way to cope with the phenomenon called "phase jump". Suppose a case in which a photoacoustic signal is detected from a sample 200 having a crack 109 in the surface, as shown in FIG. 3A, for example. FIG. 3B shows a longitudinal sectional view taken along the line A--A' in FIG. 3A, while FIG. 4A shows a phase shift image (hereafter referred to as the phase image) of two-dimensional photoacoustic images of the sample 200. In the phase image, the thin white region corresponds to the crack 109, and in this region, there are several dark areas where the phase shift occurs sharply. FIG. 4B shows a phase shift signal 111 in a section taken along the line B--B' in FIG. 4A. As is apparent from this figure, there is a sharp phase jump in the dark areas, and in those areas, the phase signal 111 shows a sharp drop and rebound.
Generally, out of various photoacoustic signals, the phase signal has a characteristic that when the phase signal exceeds +.pi., the phase changes -2.pi. relative to the phase value, and similarly, when the phase signal exceeds -.pi., the phase changes 2.pi. relative to the phase value. In this patent application, those changes are defined as the so-called "phase jump". This phase jump phenomenon occurs in the extraction of phase shift from the photoacoustic signal. More specifically, in the lock-in amplifier 16 in FIG. 1 the photoacoustic signal (interference intensity signal) is separated into a cosine component X and a sine component Y as shown by the expressions (1) and (2). The amplitude A and the phase .theta. of the photoacoustic signal can be obtained by the expressions (3) and (4). ##EQU2##
As shown in the expression (4), the phase shift .theta. can be obtained as an arctangent of a rate of the sine component Y to the cosine component X. As is well known, the principal value of the arctangent in this case exists in the range of +.pi. to -.pi.. Therefore, for example, when the phase shift .theta. has a value (.pi.+.beta.), which exceeds +.pi., the value output from the lock-in amplifier is -.pi.+.beta., that is, .pi.+.beta. added with -2.pi.. Likewise, when the phase shift has a value (-.pi.-.beta.) less than -.pi., the output value is .pi.-.beta., that is, -.pi.-.beta. added with +2.pi.. In this way, a phase jump occurs. Therefore, when a phase jump has occurred at a sampling point, the real situation is that the phase shift at this sampling point does not contain a correct phase shift information, and for this reason, information relative to the surface and the inside of the sample cannot be obtained securely.